I have a Rinnai tankless on-demand water heater. The unit is making a high-pitched whistling noise.
-Unplugged the unit from the electrical wall socket.
-Turned the gas valve to the off position.
-Turned the cold water intake line valve to the off position.
-Turned the hot water outlet valve to the off position.
-I have removed the front panel and used compressed air to blow most of the dust off of the unit.
The unit has been in this state for an hour and It still whistles. Why?